695 



Chemistry. — "On gas equilibria, and a test of Prof, van dkr 

 Waals Jr.'s fonnula." I. By Dr. F. E. C. Scheffer. (Communi- 

 cated by Prof. J. D. van der Waals). 



(Communicated in the meeting of Sept. 26, 1914). 



1. Introduction. 



It may be supposed as known lliat (he situation of the equilibrium 

 of a gas reaction at a definite temperature can be calculated, when 

 at (hat temperature we know the energy of reac(ion and the variation 

 in entropy free from concentration for molecular conversion according 

 to the chemical equation of reaction. Bo(h quantities are algebraic 

 sums of the energies and entropies of the reacting gases separately, 

 in which the terms referring to substances of different members of 

 the equation of reaction iiave opposite signs. Energy and entropy 

 of a gas free from concentralion are pure functions of the tempe- 

 rature ; in the expression for the "eqiiilihrium constant" as function 

 of the temperature the transformation energy and the change in entropy 

 free from concentration at one definite temperature and the tem- 

 perature coefficients of both occur as constants. The transformation 

 energy of a great number of reactions may be directly derived from 

 Berthelot and Thomsen's tables ; (he temperature coefficients are in 

 simple relation with the specific heats, and for this a great number 

 of data are found in the literature ; the transformation entrop}^, how- 

 ever, is generally not determined directly, but from the chemical 

 equilibria by the aid of the above expression. 



If one purposes to calculate the chemical equilibria from caloric 

 data, one will have to apply instead of the said mode of calculation 

 of the transformation entropy either direct determinations, or another 

 mode of calculation, in which exclusively quantities of the gases 

 separately are used. The determination of electromotive forces of gas 

 cells might be counted among the direct measurements ; for the tem- 

 perature coetTicient of the electromotive force is a measure for the 

 transformation entropy. But this method cannot be applied for a 

 great number of reactions e.g. for dissociations in molecules or atoms 

 of the same kind. 



Hence the calculation of chemical equilibi'ia will succeed only when 

 we have a method at our disposal to calculate the entropies of the 

 gases ' separately or the algebraic sum of the entropies of a gas equi- 

 librium from the constants of the substances. 



The thermodynamic entropy of a gas is a quantity, which through 

 its definition is determined except for a constant, and it is therefore 



46 



Proceedings Royal Acad. Amsterdam. Vol. XVII. 



